Photomultiplier tubes (PMTs) are photo-detectors capable of detecting low intensity illumination. As such, PMTs are used in many low intensity applications including, but not limited to, cell imaging, biodiagnostic instrumentation, semiconductor wafer inspection, particle counting, nuclear medicine, radiation detection, quantum cryptography, and low light imaging (e.g. security cameras or light detection and ranging (LIDAR) systems). PMTs have been used to detect low-energy photons with wavelengths from infrared (IR) range to the ultraviolet (UV) range as well as high-energy photons such as X-rays or gamma rays.
A PMT generally includes a photocathode, a plurality of dynodes (making up an electron multiplier), and an anode. When illumination impinges upon an entrance window of a PMT, photons from the incident illumination strike a surface of the photocathode and are converted into photoelectrons, which are then accelerated by a high electric field through a path delineated by the plurality of dynodes. The photoelectrons are multiplied by secondary electron emissions from the dynodes before being collected by the anode. The electrons received by the anode produce a current associated with the intensity of the incident illumination.
Depending on the configuration of a PMT and the voltages applied to the electrodes, the PMT can serve as a detector operable in a single-photon counting mode as well as in an analog or proportional mode. In addition, PMTs generally have a large sensing area, are highly responsive, and may have high signal-to-noise ratio (SNR). In some applications, PMTs are used to detect incident illumination ranging from a single photon (less than an attowatt) up to billions of photons (picowatts to nanowatts) per second.
However, existing PMTs do not meet dynamic range requirements demanded by some state of the art and emerging applications. The dynamic range is limited by the slow decay of persisting current that results from an intensity peak (i.e. the sudden rise in current due to incident illumination). As a result, existing PMTs also have inadequate sensitivity (i.e. optical responsiveness) to incident illumination that is very quickly and dynamically changing. For example, it may be difficult to detect a low intensity peak following a very high intensity peak without enough time separation for persisting current from the first (very high intensity) peak to sufficiently decay.